1. Field of the Invention
The present invention relates to the field of liquid crystal display technology, and in particular to a mixture for liquid crystal medium and a liquid crystal display using the same.
2. The Related Arts
The liquid crystal used for twisted nematic (TN) or super twisted nematic (STN) liquid crystal display is positive-type liquid crystal. The long axis of the liquid crystal molecules is parallel to the surface of the substrate when the power is off. The alignment direction of the liquid crystal molecules on the surface of the substrate depends on the rubbing direction of alignment layer, which material usually is polyimide. The alignment directions of the surface of both substrates are perpendicular to each other. Hence, the molecules within the liquid crystal layer maintain in continuous twisted alignment state from the surface of one substrate to the surface of the other substrate. After the voltage is applied, the long axis of the liquid crystal molecules will tend to be aligned along the direction of the electric field. The drawback of the TN/STN type liquid crystal display is that it has small viewing angle, brightness difference and significant chromatic aberration under large viewing angle, which needs to be improved by the compensation film and thereby increases the manufacturing cost of the display.
Multi-domain vertical alignment (MVA) TFT-LCD using negative type liquid crystal and vertically aligned film material has solved the restrictions of the viewing angle of TN/STN display. When no voltage is applied, the long axis of the liquid crystal molecules is perpendicular to the surface of the substrate. When the voltage is applied, the liquid crystal molecules will topple down and the long axis of the liquid crystal molecules will tend to be aligned along the vertical direction of the electric field. In order to solve the problems of the viewing angle, a sub-pixel is divided into multiple regions, so that the liquid crystal molecules can topple down in different direction, which makes the views of the display seen in different directions tend to be the same. There are several ways to allow the liquid crystal molecules in the different regions to be oriented in different directions in one sub-pixel. The first one is to build a bump at the upper and lower substrates of the LCD by exposure and development, so that the liquid crystal molecules around the bump can produce a certain pre-inclined angle and topple down toward the fixed direction. The second one is to form predetermined pattern of indium tin oxide (ITO) pixel electrodes at the upper and lower substrates, which will generate an electric field with a certain inclined angle to control the toppling direction of the liquid crystal molecules in the different regions. This is called as patterned vertical alignment (PVA) technology. The third one is to form ITO slits at the TFT side of the LCD substrate and full ITO at the other side. And then add the polymerizable monomer into the liquid crystal medium. First, make the liquid crystal molecules topple down by an electric field, at the same time, irradiate the monomer with ultraviolet light to polymerize and form the polymer particles which can guide the toppling direction of the liquid crystal molecules. The polymer particles deposited on the surface of the substrate play the role of the alignment. This is called as polymer stabilized vertical alignment (PSVA).
The reaction rate of the polymerizable monomer, the size and the distribution of the polymer, the surface uniformity of the substrate, and the strength of the alignment force have major impacts on the optical properties of the panel and the stability of the production. In addition to the process conditions, these factors are mainly dependent on the molecular structure of the polymerizable monomer which directly determines the speed of the light reaction, the characteristics of the polymer and the strength of the alignment force to the liquid crystal. Because the conventional liquid crystal medium comprises alkenyl compound, it is beneficial to obtain a low rotational viscosity to improve the response of liquid crystal medium. The alkenyl compound within the liquid crystal medium will affect the polymerization reaction of the polymerizable monomer easily and then the alignment of the liquid crystal medium. Hence, in general, single polymerizable monomer is very difficult to make the factors mentioned above in a favorable situation. The actual situation often attends to one thing and loses track of another, such as good uniformity of the polymer accompanied with low alignment force, or strong alignment force accompanied with slow reaction rate and so on.